1. Field of the Invention
This invention relates generally to signal detection, and, more particularly, to a method and apparatus for detecting an activation tone.
2. Description of the Related Art
In communications systems, particularly telephony, it is common practice to transmit signals between a subscriber station and a central switching office via a two-wire, bi-directional communication channel. The Plain Old Telephone System (POTS), designed primarily for voice communication, provides an inadequate data transmission rate for many modem applications. To meet the demand for high-speed communications, designers have sought innovative and cost-effective solutions that take advantage of the existing network infrastructure. Several technological advancements have been proposed in the telecommunications industry that make use of the existing network of telephone wires. One of these technologies is the XDSL technology. DSL technology uses the existing network of telephone lines for broadband communications. An ordinary twisted pair equipped with DSL interfaces can transmit videos, television, and high-speed data.
DSL systems use digital signal processing (DSP) to increase throughput and signal quality through common copper telephone wire. Certain DSL systems provide a downstream data transfer rate from the DSL Point-of-Presence (POP) to the subscriber location at speeds of about 1.5 Megabits per second (MBPS) up to 8 MBPS. The transfer rate of 1.5 MBPS, for instance, is fifty times faster than a conventional 28.8 kilobits per second (KBPS) transfer rate. One popular version of the DSL technology is the Asymmetrical Digital Subscriber Line (ADSL) technology. The full bandwidth ADSL standard is described in ANSI T1.413 Issue 2, entitled, xe2x80x9cInterface Between Networks and Customer Installationxe2x80x94Asymmetric Digital Subscriber Line (ADSL) Metallic Interface.xe2x80x9d The lower bandwidth standard, commonly referred to as G.Lite, is described in xe2x80x9cITU-T Recommendation G.992.2 (June 1999)xe2x80x94Splitterless Asymmetric Digital Subscriber Line (ADSL) transceivers.xe2x80x9d
Both T1.413 and G.Lite protocols initiate connections by sending an activation tone to the called station. The G.Lite protocol also provides for a fast retrain tone that can be sent to force a retraining of the connection. A line card at the called station detects the activation and/or retrain tones. Typically, a line card services a plurality of subscriber local loop circuits, commonly referred to as subscriber lines. The line card has interface circuitry for each line that samples the analog signal to create a digital line signal. Each subscriber line has an associated data digital signal processor (DDSP) that detects the presence of one or more of the activation and/or retrain tones. In response to receiving one of the tones, the line card establishes a data connection in accordance with the appropriate protocol.
In a typical line card serving eight subscriber lines, it is common for only one of the subscriber lines to be active with a data connection. However, even if a connection is idle, the associated DDSP and supporting circuitry are active to allow detection of activation and/or retrain tones. This significantly adds to the power load of the line card and to the amount of heat it generates. Tone detection generally results in a relatively light computational load as compared to the computational resources required to support an active G.lite or T1.413 data connection.
In accordance with industry standards, a T1.413 activation tone is a single tone at frequency bin 8 (i.e., each bin is 4.3125 kHz) with a transmit power of xe2x88x923.65 dbm for the first 64 symbols (i.e., each symbol=1/4.3125 ms) and 24 db lower for the second 64 symbols. The second 64 symbols are followed by 896 silent symbols. A G.lite activation tone consists of three simultaneous tones at bins 9, 17, and 25 with phase reversals every 16 ms. The G.lite retrain tone is a single tone at bin 20 with phase reversals every 16 ms.
The phase reversals and varied transmit power characteristics of the activation tones complicate the task of tone detection. During the phase reversal periods, the energy significantly decreases using a filter based energy detection method. If the energy of the answering tone signal is polled near or during one of the phase reversals, detection of the answering tone could be missed. If the detection threshold for detecting the answering tone is lowered to account for the possibility of polling during the phase reversal, the likelihood of a false positive detection is increased. Threshold determination is also difficult with the T1.413 activation tone due to the varied transmit power.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
One aspect of the present invention is seen in a line card for interfacing with a plurality of subscriber lines. The line card includes a plurality of data processors and a tone detector. Each of the data processors is associated with one of the subscriber lines. The tone detector is adapted to detect one of a plurality of activation tones on a selected subscriber line. The plurality of activation tones include a first activation tone having an active portion and a silent portion. The active and silent portions repeat periodically at a first frequency having a first period. A second activation tone has a phase reversal portion repeating at a second frequency and having a second period. The tone detector is further adapted to sequence between the subscriber lines at a predetermined interval. The predetermined interval is based on the first and second periods. The tone detector is adapted to signal the data processor associated with the selected subscriber line in response to detecting one of the activation tones.
Another aspect of the present invention is seen in a method for detecting activation tones on a plurality of subscriber lines. The method includes providing a plurality of data processors. Each of the data processors is associated with one of the subscriber lines. One of a plurality of activation tones is detected on a selected subscriber line. The plurality of activation tones includes a first activation tone having an active portion and a silent portion, the active and silent portions repeating periodically at a first frequency having a first period. A second activation tone has a phase reversal portion repeating at a second frequency having a second period. The subscriber lines are sequenced between at a predetermined interval, the predetermined interval being based on the first and second periods. The data processor associated with the selected subscriber line is signaled in response to detecting one of the activation tones.